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Beetle-Inspired Bidirectional, Asymmetric Interlocking Using Geometry-Tunable Nanohairs Changhyun Pang,†,§ Sang Moon Kim,†,§ Yudi Rahmawan,‡ and Kahp-Yang Suh*,†,‡ †
Division of WCU Multiscale Mechanical Design, ‡School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea S Supporting Information *
ABSTRACT: We present bidirectional, asymmetric interlocking behaviors between tilted micro- and nanohair arrays inspired from the actual wing locking device of beetles. The measured shear adhesion force between two identical tilted microhair arrays (1.5 μm radius, 30 μm height) turned out to be higher in the reverse direction than that in the angled direction, suggesting that the directionality of beetle’s microtrichia may play a critical role in preventing the elytra from shifting along the middle of insect body. Furthermore, we observed dramatic enhancement of shear adhesion using asymmetric interlocking of various nanohair arrays (tilting angle, δ < 40°). A maximum shear locking force of ∼60 N/cm2 was measured for the nanohair arrays of 50 nm radius and 1 μm height with a hysteresis as high as ∼3. A simple theoretical model was developed to describe the measured asymmetric adhesion forces and hysteresis, in good agreement with the experimental data. KEYWORDS: biomimetics, interlocking, beetle, nanohairs, dry adhesive
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electric connector,32 biomedical patch,33 and flexible, wet, or thermal responsive adhesive.29,34,35 Following our earlier observation on reversible interlocking with vertically oriented hairs,28,36 we present here bidirectional, asymmetric interlocking between tilted, high aspect-ratio (AR) micro- and nanohairs utilizing geometry-tunable replica molding and broad ion beam irradiation. In the broad ion beam irradiation, the as-formed hairy structures are irradiated by Ar ion beam at a tilting angle (0−90°) with an appropriate intensity and time, thereby changing the bending angle in a precisely controllable manner. This method is advantageous over oblique e-beam irradiation as the structural transformation is completed rapidly (30°) in the (+) direction (Figure 7a). This is presumably due to the fact that the nanohairs are rather stooped, not straight, as a result of oblique broad beam
Figure 7. Comparisons of the measured shear adhesion forces with the theory using various tilting angles in the (a) angled (+) and (b) reverse (−) directions.
irradiation.37,38 Also, the discrepancy may be associated with misalignment of upper and lower hairs and structural defects. As with the microfiber arrays shown in Figure 3, the preloaddependent shear adhesion forces were measured in the range of 0.1 and 5 N/cm2 (Figure 8a). Here, the maximum hysteresis is estimated to be ∼7 for δ = 40°, which is substantially higher 4228
dx.doi.org/10.1021/am3009289 | ACS Appl. Mater. Interfaces 2012, 4, 4225−4230
ACS Applied Materials & Interfaces
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Research Article
SUMMARY We have presented bidirectional, asymmetric interlocking between tilted micro- and nanohair arrays inspired from the actual wing locking device of beetles. The measured shear force turned out to be higher in the reverse direction than that in the angled direction, suggesting that the directionality of tilted microtrichia may play a critical role in preventing the elytra from shifting along the middle of insect body. A maximum shear locking force of ∼60 N/cm2 was observed for the nanohair arrays of 50 nm radius and 1 μm height (SR = 3) with a hysteresis as high as ∼3. A simple theoretical model was developed to describe the measured asymmetric adhesion forces and the hysteresis, which were in good accordance with the experimental data.
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ASSOCIATED CONTENT
* Supporting Information S
Calculation of Hamaker constants, detailed derivation of maximum angular displacement of hairs Δθm, and supplemental figures (Figures S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +82-2-880-9103. Fax: +82-2-883-1597. E-mail: sky4u@ snu.ac.kr. Author Contributions §
These authors contributed equally to this work.
Figure 8. (a) Plots of preload dependency of shear adhesion force with 40° stooped−40° stooped and vertical nanofiber arrays. (b) Comparison of the measured adhesion hyteresis with the theory.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant (No. 20110017530), the WCU (World Class University) program (R31-2008-000-10083-0), the and Basic Science Research Program (2010-0027955). This work was supported in part by a Korea Research Foundation Grant (KRF-J03003) and the Global Frontier R&D Program on Center for Multiscale Energy System.
than the measured value of ∼3 (Figure 8b). This discrepancy seems also attributed to the same reasons mentioned above (stopped geometry, misalignment, structural defects, etc). The durability of the current bidirectional interlocker was independently assessed, in which no notable reduction in the measured shear force was observed over 10 repeating cycles of attachment and detachment (Figure S4, Supporting Information). After that, the force was slowly deteriorated due to the continuous collapse and mating between neighboring hairs. It is noted in this regard that the mechanical strength of hairs could be substantially enhanced by coating a thin metal (e.g., Pt) layer of ∼5 nm at the expense of 10−20% reduction of the shear adhesion force.37 We also measured pulling forces with three different combinations of tilting angles in the reverse and forward directions: 40° stooped−40° stooped, vertical−vertical, 15° stooped−40° stooped (Supplementary Figure S5, Supporting Information). We found that the shear adhesion force between 15° stooped−40° stooped nanohairs was slighlty decreased as compared to 40° stooped−40° stooped hairs, probably due to the reduction of overlapping length between upper and lower hairs. To further demonstrate the utility of asymmetric adhesion, two pendulums (3 kg in − direction and 1 kg in + direction) were supported by 0.8 × 0.8 cm2 patches with different pulling directions (Supplementary Figure S6, Supporting Information). Also, this asymmetric, bidirectional adhesion may be utilized as a simple, smart fastener to fix items with moderate weight when two different pulling forces are needed in a controllable fashion (Supplementary Figure S7, Supporting Information, with snapshots).
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